Storm Forecast
Valid: Fri 11 May 2007 06:00 to Sat 12 May 2007 06:00 UTC
Issued: Fri 11 May 2007 03:04
Forecaster: VAN DER VELDE

SYNOPSIS

Between shallow low pressure over Scandinavia/northern Atlantic and weak high pressure over the Mediterranean, tight pressure gradients are present over much of western and central Europe. Within this strong zonal flow pattern a low pressure area/trough with associated wave moves from the Netherlands to western Russia during this forecast period. Instability in the warm sector has proven to be increasingly available over central Germany during last night as low level warm air advection and adiabatic cooling of mid levels by QG lift proceeds.

At the warm side of the cold front most instability will develop, generally a few hundred J/kg CAPE but perhaps locally up to 1000 J/kg.
The frontal zone is forecast to stall over northern Italy into Hungary.

From the Atlantic, elevated instability (almost no 0-1 km parcel signals, but GFS convective precipitation and cold MU-EL tops) will advect to the British Isles and possibly as far as the Netherlands late in the period.


DISCUSSION

...Poland, Belarus area...

Strongest synoptic scale forcing will be around the occlusion dragging over northern Poland, where a mix of rain and embedded convective elements will likely be present during the day. A jet maximum over the cold front spanning Poland from southwest to northeast during the time of diurnal heating (...thought to indeed take place) creates strong deep layer shear (>30 m/s 0-6 km, >20 m/s 1-8 km) through the prefrontal area of CAPE. Additionally, more than 15 m/s of vertical shear between 0-1 km is forecast by GFS... and 200-300 m2/s2 SREH over 0-3 km, especially due to backed (southeasterly) flow near the surface. LCL heights are predicted to be in the 1000-2500 m range, not particularly low.

Given these conditions, it is thought that supercells will be the common mode for the Poland area, with the best chances just south of the occlusion in the warm sector, adjacent to more large scale rain in the north. As the cold front does not show strong gradients and no sharp convergence zones are indicated, an MCS may not be the main mode until the evening.
Diurnal heating will gradually enhance the number of storms and their severity, and the level 2 has been placed over northeastern Poland where the warm tongue, shear, lift and diurnal heating are thought to be maximized. Surrounding the level 2, more isolated potentially severe storms, possibly also supercells, can occur.

Within the strong wind environment, and relatively high LCLs, severe gusts (>25 m/s) may be produced with the stronger storms. Large hail is likely. A few tornadoes can be expected with supercells in this strong low level shear/helicity situation, most probability in the zone of lower LCLs in northern Poland, perhaps also southern Lithuania, areas where 10m vectors may back most.
During the evening storms may group into an MCS and affect mostly Belarus, with decreasing hail/tornado threat but remaining severe gusts threat.


...Slovakia, Hungary, Austria, northern Italy...

Higher CAPE is forecast over Hungary, while the good deep layer shear is only touching upon the western edge of the tongue of unstable air. Still more than 15 m/s deep layer shear and >100 m2/s2 SREH is forecast to be present, and abundant convective precipitation signals. Long-lived multicells, perhaps isolated cases tending to become supercells can be expected with a large hail chance, especially near Hungary where there is a bit more CAPE to work with, but the lee of the Alps may also see isolated hail.